As a result of an advance in information communication technologies and various demands of an information-oriented society, there is an increasing demand for display devices, and flat panel display (FPD) devices are being widely used. For example, an electroluminiscent display device (ELD) device, a liquid crystal display device (LCD) (e.g., a TFT-LCD or TN/STN) device, a plasma display panel device, and an organic EL device are being used as flat panel display devices. To stably control product quality of flat panel display devices, it is conventional to detect defects and remove them from fabricated flat panel display devices. For example, when pollutants remain on a flat panel display device, they may lead to defects, such as bright dot, dark dot, hot pixel, dead pixel, or the like. The defect sources on a flat panel display device can be examined by an optical inspection process. For example, a laser beam can be used for such an optical inspection process. However, in the case where a display substrate may allow the laser beam to penetrate therethrough, the laser beam may be scattered by a substrate supporting structure disposed under the display substrate. A thin film may be coated on the display substrate to reduce optical transmittance of the display substrate, but it may be difficult to prevent the laser beam from passing through the display substrate. In other words, there may be difficulty in preventing the laser beam from being scattered by the substrate supporting structure. In the case where the optical inspection process is performed using light scattered by the display substrate, light scattered by the substrate supporting structure may serve as a noise factor in the optical inspection process, thereby causing deterioration in image quality and in inspection reliability of the optical inspection process.